1. Field of the Invention
The present invention relates to a hot melt ink transfer recording sheet and a process for producing the same. More particularly, the present invention relates to a hot melt ink transfer recording sheet which exhibits a high resistance to degradation of appearance, for example, caving formation of indents in the form of spots or stripes of the recording sheet, and thus is appropriate for hot melt ink thermal transfer printers in which the recording sheet is brought into contact with a thermal head of the printer through a hot melt ink ribbon under a high contact pressure; which can accurately receive a plurality of differently colored images at the desired recording positions without deviating the positions of the different coloring ink dots, and thus is useful for multi-color printing systems in which a plurality of different colored images are repeatedly transferred from the coloring ink ribbons; and which can record thereon colored images having excellent color density, a high gradation reproducibility and a superior dot reproducibility, and a process for producing the same.
2. Description of the Related Art
It is well known that a hot melt ink thermal transfer recording system using a hot melt ink transfer recording sheet and a thermal head of a thermal transfer printer has a simple mechanism and can be easily maintained, and thus is widely utilized in the printers for word processors and the printers for labels. In the hot melt ink thermal recording system, woodfree paper sheets have been mainly employed as the hot melt ink recording sheets. However, in the recent trend, full colored images with a high quality have been strongly in demand in ink jet recording system, dye-sublimation transfer recording system, laser recording system, etc.
There have been various attempts for full colored image-printing in the hot melt ink thermal transfer recording system. With respect to the printer, the conventional system in which a desired gradation of the full colored images is attained without changing the size of the transferred ink dots is replaced by a newly developed system in which a printer capable of varying the size of the unit dots, namely, a variable dot printer, is used. For example, the G6800-40 Printer made by MITSUBISHI DENKI is of the variable dot type. Also, the hot melt ink thermal transfer printer requires that the hot melt ink transfer recording sheet has such an important property that, in a full color recording with a wide range of applied printing energy from a low level to a high level, the hot melt-transferred ink dot forms can be accurately reproduced on the recording sheet, namely the dot reproducibility is high, and the ink can be transferred in a sufficient amount from the ink ribbon to the recording sheet, namely the color density of the recorded ink images is high.
In view of the above-mentioned technical background, the hot melt ink transfer recording sheet must be appropriate to the above-mentioned specific performance of the printer. For example, when a non-coated paper sheet for usual printing is used in the variable dot type hot melt ink transfer printer, the transferred ink images may have an unsatisfactory color density which may be derived from the low thermal insulating property of the non-coated paper sheet, and an insufficient dot-reproducibility which may be due to a low cushioning property of the non-coated paper sheet. Also, when the recording surface of the non-coated paper sheet is rough, the colored images may have no-ink-printed spots. These phenomena cause the dot-reproducibility to be poor. In addition to the reduction in the color density of the recorded ink images due to the poor dot-reproducibility, a further reduction in the color density of the recorded ink images may occur due to a low ink-absorption of the hot melt ink-receiving layer.
As an attempt to solve the above-mentioned problems, Japanese Unexamined Patent Publications No. 2-89,690 and No. 64-27,996 disclose an undercoat layer formed on a surface of the substrate sheet and comprising hollow solid particles. The resultant hot melt ink transfer recording sheet was, however, unsatisfactory in the cushioning property and heat insulating property enhancing effect thereof. Also, the recording sheets of the Japanese publications were disadvantageous in the following items. Namely, when the hollow solid particles are soluble in an organic solvent contained in a coating liquid for the ink-receiving layer, it is necessary that the hollow solid particles are bound with a binder comprising a specific polymeric material having a high resistance to the organic solvent or that an additional polymeric material layer having a high resistance to the organic solvent is formed on the undercoat layer containing the hollow solid particles, and thus the production of the recording sheet is complicated.
As another attempt to solve the afore-mentioned problems, Japanese Unexamined Patent Publication No. 2-41,287 discloses a recording sheet prepared by forming a resin layer comprising a water-soluble component, which can elute into water, on a substrate sheet comprising, as a principal component, a plastic resin; elution-removing the water-soluble component from the resin layer to form fine pores in the resin layer and to thereby enhance the ink-absorption capacity of the resultant hot melt ink transfer recording sheet. This attempt was, however, not fully successful because the maximum color density of the ink images recorded on the recording sheet was unsatisfactory, or the gloss of the printed ink images was insufficient, and thus the resultant recording sheet does not fully meet with the requirement for the qualities of the hot melt ink transfer recording sheet. Also, this type of the recording sheet is disadvantageous in that the substrate sheet thereof comprises, as a principal component, a plastic resin, and thus the recording sheet is difficult to recycle after use.
The conventional printers, in which the size of the image dot is not variable and a conventional type of dot are used, include a type of printer in which, when a thermal head of the printer is brought into contact with a recording surface of a recording sheet through an ink ribbon, the contact pressure of the thermal head is designed to be high, to make sure the transfer of the imagewise ink dots from the ink ribbon to the recording sheet surface and to thereby meet with the requirements for the good dot reproducibility, the high color-gradation reproducibility and the high color density of the recorded images. This type of printer includes a microdry-type printer, for example, the printers available under the trademark of PRINTER MD-1000, MD-1300 and MD-2000J, from ALPS DENKI K.K. The microdry type printers are advantageous in that the contact pressure of the ink ribbon with the recording sheet surface in the ink dot-transferring procedure is high, and thus the recording sheet does not need a high cushioning property and a high thermal insulating property to obtain a high quality of recorded ink images, and thus are definitely distinguished from the variable dot type printers. The contact pressure of the thermal head of the microdry type printer with the ink ribbon is assumed to be several tens kg/cm.sup.2, while the contact pressure in the variable dot type printer is assumed to be several kg/cm.sup.2. Also, currently, a new type of hot melt ink transfer printer has been developed by modifying the variable dot type printer so that an advantage that the contact pressure of the thermal head of the printer with the hot melt ink transfer recording sheet, through the ink transfer ribbon, is imparted to the variable dot type printer. In this type of printer, a very high quality of full colored ink images has a very good dot reproducibility over the low to middle color density range and a very high color density of the images over the high color density range. This type of printer includes, for example, a printer available under the trademark of PRINTER MD-5000, from ALPS DENKI K.K.
Usually, woodfree paper sheets or specific coated paper sheets comprising a substrate paper sheet and a hot melt ink-receiving layer formed on the substrate paper sheet and containing a certain type of pigment are used as hot melt ink transfer recording sheets for the printers which employ a high contact pressure of the thermal head. In this case, the transferring property of the hot melt ink to the recording sheet is not always sufficient in the recorded images in the low to middle color density range, and thus the above-mentioned conventional recording sheets cannot fully meet with the industrial demands which require the high quality of ink images. Also, since the contact pressure of the thermal head is high, the substrate sheet of the recording sheet is elongated by the first ink dot transfer procedure in the direction in which the thermal head scans, and thus due to the dimensional changes of the recording sheet, the second and later transferred ink dots cannot be accurately superposed on the first transfered ink dots. Therefore, the resultant colored images formed from a plurality of single colored ink images superposed on one another may have an unsatisfactory accuracy and differently colored tone.
Further, Japanese Unexamined Patent Publications No. 7-309,074 and No. 8-282,137 discloses a hot melt ink transfer recording sheet having a porous ink-receiving layer formed on a surface of a substrate sheet from a bubbled resin coating liquid. This type of the recording sheet is, however, disadvantageous in that, when the recording sheet is used in the printer in which a high contact pressure of the thermal head is applied to the recording sheet, the image-transferred portions of the recording sheet are indented by the high contact pressure of the thermal head, and thus the appearance of the recorded sheet is degraded.